1. Field of the Invention
This invention relates to fluidization aids.
2. Description of the Prior Art
Gas-solids fluidization is a process where gas flows upwards through a bed of solid particles at a sufficient velocity that the particles begin to move and form a gas-solids suspension with properties like a liquid (hence the term "fluidized"). The advantages of these liquid-like properties are then employed to handle the particles and to carry out gas-solids chemical reactions, gas-phase solid-catalyzed chemical reactions or physical operations such as drying or coating. Traditionally, only particles larger than approximately 30-40 .mu.m in diameter can be well fluidized. Finer and/or lighter particles, known as Geldart Group C or cohesive particles, are very difficult to fluidize. Most Group C particles are smaller than 30 .mu.m, but there are exceptions.
The reason for the difficulties in fluidizing Group C fine powders is that there are relatively large inter-particle forces between fine particles, which reach or exceed the magnitude of the weight-minus-bouyancy force which must be overcome to fluidize the particles. Various methods have been applied to aid the fluidization of fine particles. Those methods include applying vibration, acoustic (including ultrasonic) waves, mechanical stirring, electromagnetic forces, or the addition of a particulate "fluidization aid" such as silica or anti-static agents to help to overcome the inter-particle forces.
The existing methods have several drawbacks. Firstly, the existing methods are not always effective. They tend to be more effective for certain fine powders than for others. In general, if the inter-particle forces are too strong, the aids are simply insufficient to overcome them. In addition, none of the existing methods work for extremely fine and cohesive powders, e.g. for most particles smaller than approximately 10 .mu.m in diameter. Secondly, they require extra components, such as vibrators, acoustic field generators, motors, etc., to generate the external aids. Thirdly, they require additional energy to provide the external aids, or they require addition of foreign particles which may contaminate the product.
To obtain better fluidization, it is known to use fluidization aids, in addition to the fluidizing gas. Such aids include the addition of other powders such as antistatic aids or hydrophobic silica, mechanical means such as stirring, agitation or vibration of the bed, acoustic (including ultrasonic) waves, electromagnetic forces, etc. These aids help to overcome the inter-particle forces and produce better fluidization than is possible with the fluidizing gas alone.
In the prior art, such as in U.S. Pat. No. 3,639,103 (Sheely) for example, inert gases including helium have been used as fluidizing gases, so that they would not participate in a chemical reaction. However, the prior art teaches that, particularly when it comes to fine powders, fluidization aids are required regardless of what the fluidizing gas is.
U.S. Pat. No. 5,277,245 (Dutta et al.) uses a fluidizing gas comprising helium, hydrogen and mixtures thereof, optionally in combination with other fluidizing gases, to enhance the heat transfer characteristics of fluidized beds of Geldart type C powders. However, Dutta et al. blends the powder with a fine particle size hydrophobic silica, which functions as a fluidization aid.
U.S. Pat. No. 5,258,201 (Munn et al.) discloses a method for forming a protective coating on phosphor particles used in the manufacture of fluorescent lamps. One step in the process comprises the suspension of phosphor particles in a fluid zed bed. An inert gas, helium being one example, is passed upwardly through the fluidized bed. However, for fluidizing fine particles, a small amount of a fluidization aid is mixed with the phosphor powder, or it is stated that "fluidization of fine phosphor powders can alternatively be accomplished by additional agitation of the phosphor powder particles . . . ".
Three other U.S. Pat. Nos. 4,585,673; 4,710,674 and 4,825,124 (all Sigai), are very similar, and also contain similar statements regarding the need to add fluidization aids when fluidizing fine powders.
U.S. Pat. No. 5,783,721 (Tsumura et al.) relates to a process for the preparation of alkylhalosilanes by reacting fluidized particles of metallic silicon powder with a gaseous alkyl halide in the presence of a copper catalyst (also in the form of fluidized particles) in a fluidized bed reactor. Difficulties in fluidizing Geldart type C powders are discussed. These difficulties are overcome by carefully controlling the size distribution of the particles to ensure that there are sufficient larger-sized particles to act as fluidization aids for the smaller particles. The fluidizing gas is an inert gas, preferably nitrogen, although it is stated that helium or argon could be used.
Note that there is an important distinction between fluidizing gases and fluidization aids. In fluidized beds, the gas which is flowed through the bed of powder and is primarily intended to lift or expand the bed of powder, is the fluidizing gas. Helium in particular has been used as a fluidizing gas, selected for its inert nature or for its high thermal conductivity, but helium has not been previously recognized as a fluidization aid.